Second couple held over gas bottles found near Notre Dame

French police were searching Thursday for two "radicalised" women linked to a car containing six gas cylinders found in central Paris, with four people already held as part of the investigation.

The street where the gas cyllincers were found France 24

Speaking on Thursday, President Francois Hollande referred to attack plots that been foiled "in recent days", without elaborating.

A first couple, held since Tuesday, are known to the security services for links to radical Islamists.

The two women being sought are the daughters of the car's owner -- who was also known to authorities for spreading Islamist ideas. He was arrested on Tuesday but released later that day.

The daughters have both been described as "radicalised".

Anti-terror investigators are probing the incident which comes with France on high alert following a string of jihadist attacks, including November's coordinated Islamic State (IS) group assaults in Paris by gunmen and suicide bombers that killed 130 people.

Police picked up the second couple overnight near Montargis, about 110 kilometres (65 miles) south of Paris, a police source said.

The first couple, detained on Tuesday at a service station on a road in southern France, were known to the security services for links to radical Islamists. Both of them remain in custody.

The car's owner, also known to authorities for spreading Islamist ideas, was also taken in for questioning but released on Tuesday evening

No number plates

A bar employee working near Notre Dame raised the alert on Sunday after noticing a gas cylinder on the back seat of the car, a police source said.

That cylinder was found to be empty but five full cylinders were found in the boot of the car.

Police found no detonators in the vehicle.

Photographs of the metallic silver-coloured car after it was discovered showed its boot open and the gas canisters placed on the ground in a quiet side street opposite the cathedral.

Notre Dame, renowned for its flying buttresses, stained glass windows and gargoyles, is one of Paris's most popular landmarks, attracting 13 million visitors each year.

Speaking on Wednesday, Interior Minister Bernard Cazeneuve said the intentions of those arrested were as yet unknown.

IS said the truck was driven by one of its followers.

Less than two weeks later, two young jihadists murdered a priest near the northern city of Rouen.

Back in May, the head of France's DGSI domestic intelligence service, Patrick Calvar, warned in May of a "new form of attack" in which explosive devices would be left near sites that attract large crowds.

French security services are particularly worried about the danger posed by extremists returning from Syria after fighting with IS forces, with 700 French nationals still in the country, according to France's top prosecutor